A directional coupler is in principle a reciprocal, loss-free four-port structure, in which each port is decoupled from one of the three other ports. Directional couplers, and the design and use of said directional couplers are described, inter alia, in “Taschenbuch der Hochfrequenztechnik” [“Pocketbook of radiofrequency engineering”], 4th edition, 1986, Springer-Verlag. The directional coupler may be formed by discrete components. Furthermore, the directional coupler may also have, at least partially, line sections instead of discrete components. The directional coupler is in principle a component part in radiofrequency engineering and is used, inter alia, to branch off from a waveguide or a line some of the energy from the electromagnetic waves guided therein in directionally dependent fashion. The technical design is dependent, in particular, on the frequency of the electromagnetic waves applied to the directional coupler.
One application area for directional couplers is in signal monitoring and/or matching monitoring of transmitters and in the measurement of a standing wave ratio, for example. By a directional coupler, signals based on electromagnetic waves may be coupled out of the waveguide separately according to their propagation direction. An important application area for directional couplers is magnetic resonance tomography devices. In this case, the directional couplers are used for distributing and measuring radiofrequency electromagnetic waves. Directional couplers are made to specification in small numbers, in particular, in the high-power range, and are therefore correspondingly complex to manufacture. The directional couplers require a large amount of installation space for high powers and are therefore expensive.
In principle, the directional coupler has four ports, to which lines or further functional modules may be connected. An important property of a directional coupler is that an electromagnetic wave, which is supplied at one of its ports, splits with a defined ratio at two functionally opposite ports and is not coupled out at the further port on the feed-in side. This property applies, in principle, to any port of the directional coupler.
Directional couplers whose four ports are coupled by a transformer that has three windings are known. The use of the transformer has the disadvantage that, in this case, not always the same characteristic impedance is available at the ports of the directional coupler. In the case of radiofrequency circuits, this is desirable, however. Although the characteristic impedance may be matched by matching of the transformation ratio of the windings with respect to one another, such a winding may not be wound in trifilar fashion, which results in further problems, in particular, in respect of the coupling factor. Furthermore, a problem with respect to the value of the characteristic impedance remains at one or more of the ports. Correspondingly, circuitry complexity is provided in order to be able to match the characteristic impedance by supplementary matching networks, e.g., such that all four ports have the same characteristic impedance.
Furthermore, it is known to form the abovementioned transformer by two line transformers in order to be able to transmit in particular high frequencies with low losses and with a particularly wide bandwidth. However, the problem of non-uniform characteristic impedances at the four ports remains in this case too.
Furthermore, special directional couplers, namely ring couplers, (also referred to as rat race couplers), are known. The couplers have a particularly narrow bandwidth, wherein individual line segments may be formed by lines of discrete elements. For such a ring coupler, at least 10 elements are required, namely at least four inductances and six capacitances.